A pre-plasticizing type injection molding machine is illustrated in FIG. 14. This machine consists of an extruding machine 3 provided with a screw 2 and has a plasticizing and feeding function while measuring. An injection pot 6 inside of a metal barrel 1 is coupled to the extruding machine 3 via a passage 5 with a check valve 4. A plunger 7 reciprocates in the injection pot 6, a part 8 positioned at a tip of the pot 6 serving as an injection passage (hereinafter, “a tip part of the injection pot”). An injection nozzle 9 is provided at a tip of the tip part 8 of the injection pot inside the metal barrel 1, where a lower portion of the metal barrel 1 positioned near its lower end is supported by an upper die plate 11 on a mold 10. Here, reference numeral 12 denotes a lower die plate coupled by the upper die plate 11 and tie bars 13, reference numeral 14 denotes an insulating plate, and reference numeral 15 denotes a lower mold also serving as a heater plate.
In the injection molding machine, first, raw rubber is supplied into the extruding machine 3 as indicated by an arrow in FIG. 14, it is plasticized while being fed to the left in FIG. 14 by the screw 2, and the plasticized rubber is fed into the injection pot 6 by the passage 5 via the check valve 4, while measuring, and the plunger 7 is elevated by the feeding pressure of the plasticized rubber. Next, by pushing down the plunger 7, plasticized rubber inside the injection pot 6 is fed into the die 10 via the tip part 8 of the injection pot and the nozzle 9 and is injected into cavities 19, 20 in the mold 10 via a sprue 16, a runner 17, and a gate 18, where curing is conducted.
In order to improve production efficiency of a cured rubber product, and to reduce product cost, it is effective to reduce the curing time for the plasticized rubber in the cavities 19, 20. To do this, the temperature of the rubber in the injection pot 6 is required to be set as high as possible. However, when the temperature of plasticized rubber in the injection pot 6 is made excessively high, “an initial stage were a curing reaction starts so that flowability is lost”, referred to as scorching, occurs easily, so that scorched rubber advances into the cavities 19, 20, which may result in product failure or clogging at the tip part 8 of the injection pot or the nozzle 9.
Control of plasticized rubber temperature is important for avoiding the above circumstances. In this context, an injection molding machine such as that illustrated in FIG. 14, is convenient. In this machine, plasticized rubber is fed into the injection pot 6 from a side part of the pot via the passage 5, after being plasticized and metered within the extruding machine 3, and the rubber temperature after heat generation due to the plasticization in the extruding machine 3 and the heat-retention temperature inside the injection pot 6 can be controlled independently.
Incidentally, in an injection molding machine where a screw extruding machine is incorporated, in a plunger-type injection molding machine such as that disclosed in Japanese Patent No. 3174346, to be described later, the rubber temperature after heat generation inside the screw extruding machine and the temperature inside the injection pot influence each other, resulting in difficulty in controlling both temperatures separately from each other. In a commercially-available injection molding machine of one kind, where a plasticized material from an extruding machine is fed into the pot from a nozzle at a tip part of the injection pot, an attempt has been made to utilize the injection heat generated by the small diameter of the nozzle for reducing the curing time. However, this design requires a great deal of time for feeding the plasticized material into the injection pot.
In the machine shown in FIG. 14, the generation of scorched rubber is further accelerated when rubber adheres to an inner wall face, such as that of the injection pot 6, and remains at that place for a long time. It is therefore desirable to reduce as much as possible the number of mold injections required for replacing the plasticizing rubber present in the injection pot 6 with fresh-fed plasticizing rubber. This shortens the time the rubber remains adhered to the inner wall face in order to suppress generation of scorched rubber.
That is, it is desirable that rubber in the injection pot or the tip part be injected completely into the molds and be replaced by fresh plasticizing rubber in the fewest possible number of injections, but when the number of injections required to inject all of the rubber in the injection pot increases, the replacing performance is poor. Further, when the temperature in the injection pot 6 is elevated, scorching occurs easily. On the other hand, when the replacing performance is excellent, and the number of mold injections needed to inject all the rubber in pot 6 can be reduced, then the time during in which plasticizing rubber is overheated on the inner wall face of the pot is shortened. As a result, the generation of scorch can be suppressed.
In the general injection molding machine shown in FIG. 14, since the tip part 8 of the injection pot has the same diameter over its entire length, and the flowing velocity of plasticizing rubber is considerably reduced at its inner wall face as compared with the central part of the rubber at the time of injection, plasticizing rubber tends to remain on the walls, like cholesterol, when an injection is completed. In order to remove the residual rubber from the tip part 8 of the injection post as preparation for replacement by fresh plasticizing rubber, the injection must be repeated many times (at least six times).
When residual rubber accumulates at the tip part 8 of the injection pot over a period of time, so that the inner diameter (flow diameter) of the passage 8 becomes small, removal cannot be achieved just by repetition of the injection. In such a situation, the residual rubber may be generally removed, for example, by detaching the barrel 1 from the upper die plate 11 to clean the tip part 8 of the injection pot, the injection nozzle 9, and the like. This is usually done after a number of injections, obtained experimentally, and requires much time and labor and causes an increase in cost.
In order to solve this problem, it has been suggested that by gradually reducing the diameter of the tip part of the injection pot toward the nozzle, and setting the inclination angle of the inner wall face of tip part of the injection pot relative to the axial line of the injection pot to 0.2° or more (in a section including the axis), like the injection molding machine describe in Japanese Patent Application Laid-open No. H10-166403, a flow rate distribution of plasticizing rubber at the time of injection is changed to improve the flow rate at the inner wall part of the tip part of the injection pot to prevent the residue of plasticizing rubber at the time of injection completion. However, when clogging does occur in the shape shown in the above application, the problem of the clogging cannot be solved by a repetition of the injection, and the clogged rubber has to be taken out by disassembling the injection molding machine, or part replacement has to be conducted, so that the time and cost therefor are further increased as compared with the above-described case.
Briefly, in a conventional, common injection molding machine, since plasticizing rubber tends to easily remain on the inner wall face in the tip part of the injection pot or like structure, the number of times an injection must be repeated in order to replace the residual rubber is increased beyond necessity. Besides, when the residual rubber cannot be removed even by injections for replacements, it is necessary to detach the barrel to perform cleaning, and the time and cost therefor are significant. Further, in injection molding machines where the tip part of the injection pot is gradually reduced toward the nozzle, which is described in the above application, once clogging occurs in the passage, disassembling of the injection molding machine, part replacement therein or the like is also required. The occurrence of such problems means that the rubber within the injection pot must be set to a temperature within the range in which scorching does not occur, with the result that an improvement in production efficiency and reduction in product cost cannot be achieved.
In view of these circumstances, the present inventor previously made an invention, disclosed in Japanese Patent Application Laid-open No. 2001-237148, that has, as a main object, the removal of residual rubber effectively without cleaning or disassembling the machine while reducing the number of times of injection for replacing the residual rubber to as few as possible. That application is directed to an injection molding machine where an injection pot is provided with a feeding port for supplying a formed material in a plasticized state into the pot. A plunger is slidably disposed in the injection pot, and a tip part of the plunger is formed in such a shape as to fill a tip part of the injection pot at the time of injection.
For example, as illustrated in FIGS. 12 and 13, a tip part 22 of an injection pot 24 in a barrel 21 is formed in a shape having a tapered section leading directly from a lower end of the injection pot 24 body part. An inner face of an injection nozzle 23 is also formed with an inner wall 23a having a tapered section which is continuous with the tip part 22 of the injection pot. A tip part 27 of a plunger 26 is formed to conform with the shapes of the tip part 22 of the injection pot and the inner wall 23a of the injection nozzle 23. The angle θ of the tapered selection depends on the diameter of the injection pot 24 and is determined according to the size of the injection molding machine, the thickness of the upper die plate 11, and the position of the extruding machine 3 mounted on the barrel 21. The taper angle θ is set to 30° in the illustrated example.
According to the above construction, plasticized rubber is fed by the extruding machine 3 into the injection pot 24 through a feeding port 25, via a passage 5. The rubber in pot 24 is pressed into the tip part 22 of the injection pot by the descent of plunger 26, and is forced into cavities 19, 20 in the mold 10 via the nozzle 23 and is cured. After a predetermined descending of the plunger 26, the tip part 27 thereof advances into the tip part 22 of the injection pot so that the plunger 26 descends while continuously injecting rubber from the injection nozzle 23. At this time, the flow rate on the inner wall face becomes large, owing to the shape of the tip part 22 of the injection pot, so that rubber on the wall face flows down without delay. Since the tip portion 22 of the injection pot is formed in a tapered shape, the junction, or a connecting point K between the tip part 22 of the injection pot and the injection pot 24 approaches a straight line, and the angle between the tip part and the pot constitutes an obtuse angle. Therefore, flowing-down of the rubber is conducted smoothly, so that the possibility of scorching is reduced.
When the lower end of the body part of the plunger 26 reaches a lower end of the injection pot 24, as shown in FIG. 13, the entire plunger tip part 27 fits into the tip part 22 of the injection pot and the projection end 27a of the plunger abuts on the inner wall 23a of the injection nozzle 23. At this time, there is hardly any residue of rubber in the tip part 22 of the injection pot or in the injection nozzle 23 in this example, so that it is unnecessary to repeat the injection cycle many times in order to remove residual rubber from the tip part 22 of the injection pot or the like as preparation for replacement by fresh plasticized rubber.